The invention relates generally to methods and apparatus for driving loads at high power levels from different power sources, and in particular, to a method and apparatus for driving an induction motor from sources having different voltages and frequencies.
European railway trains, which operate from overhead lines, and in several countries, must operate at several different voltages and frequencies. In current practice, this means that every voltage and frequency requires a separate transformer. These transformers take up nearly the entire engine compartment, and sometimes that of a second vehicle as well, because of the power requirements of the transformer are typically in the three to eight megawatt range.
In particular, traction engines in Germany use an overhead transmission line operating at 162/3 Hz, at 25 kilovolts. The engine is connected to the source of power by a pantograph which connects to an 8,000 volt-ampere power transformer, rectifier, filter, and three phase inverter which drives the induction traction motors of the train. Interestingly, however, in Germany many of the trains must meet a requirement that the traction engine operate on up to a 4% grade. This relatively steep grade adds weight to German traction engines (due to the larger motor and transformer requirement) and can put them at a severe disadvantage compared to French engines. In particular, the additional weight is sufficient to put German traction engines, which have a weight of 1.9 tons per axle, beyond the 1.7 tons per axle limit for trains allowed to operate in France. Thus, the German engines and hence the German railroad system is at a competitive disadvantage in France since they must use French traction engines to complete some journeys.
It would thus be desirable to provide a solution which reduces the weight of the German engine while at the same time continuing to enable it to operate at voltages and frequencies of different sources. Such a change in the engine compartment of the traction engine would also have equally useful effectiveness in other fields where weight and size are important, and where multiple voltage sources (for example ranging from 480 to 38,000 volts) and frequencies must be accommodated, such as large power supplies and for use with power distribution applications (particularly where the input is constant and different output voltages and configurations are needed). This problem is particularly applicable to the European railway system because of the large cumulative or aggregate size and weight of the transformers needed to operate in various countries, and in particular, as noted above, at substantial grades.